1. Field of the Invention
The present invention relates to the growth of SiC crystals having a reduced boron content.
2. Description of Related Art
Wafers of semi-insulating silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates to grow epitaxial layers of SiC and AlGaN, which are used for the fabrication of SiC- and AlGaN-based microwave devices. In order for the devices to work efficiently, the substrates must be semi-insulating, that is, fully compensated electronically. In practical terms, it is required that the resistivity of the SiC substrate must be higher than 105 Ohm·cm, and desirably above 106 Ohm·cm, and even more desirably above 107 Ohm·cm, as measured at room temperature and under normal room light.
Electronic properties of semiconductors are determined by their electronic band structure, particularly by the bandgap, which is an energy gap that separates the valence band from the conductance band and is typically expressed in electron-volts (eV). The bandgap of silicon carbide is 3 eV for 6H polytype and 3.2 eV for 4H polytype. In ideal semiconductors that contain no impurities or defects, there are no energy levels within the bandgap and their electrical conductance is determined by electronic transitions between the valence and conduction bands. Due to the large value of the SiC bandgap, the calculated electrical resistivity of the “ideal” SiC is extremely large—about 1020 Ohm·cm at room temperature.
In reality, however, semiconductor crystals contain impurities and defects which lead to the appearance of impurity- and defect-related electronic levels within the bandgap. Electronic properties of such “non-perfect” semiconductors are determined to a very large degree by electronic transitions involving these extrinsic levels in the bandgap. Impurity and defect levels lying close to the edges of the bandgap are called “shallow”. In 4H and 6H SiC, nitrogen is a shallow donor having its level at about 0.1 eV below the conduction band, while boron is a shallow acceptor with its level at about 0.3 eV above the valence band. Due to the relatively small energy gaps separating shallow levels from the band edges, they are thermally ionized at room temperature producing free charge carriers either in the conduction band (free electrons) or in the valence band (free holes). This makes the room-temperature electrical resistivity low. As an example, the resistivity of SiC crystals containing boron can be as low as 0.5 Ohm·cm.
Extrinsic levels positioned closer to the midgap are called “deep” levels. Vanadium and some point defects produce deep levels in SiC, which are at 0.8-1.5 eV from the band edges. Due to the relatively large energy gaps that separate deep levels from the band edges, they are not thermally ionized at room temperature and, therefore, do not supply free charge carriers. On the contrary, deep levels are capable of removing free charge carriers from the conductance and valence bands, thus leading to the increased electrical resistivity. This phenomenon is called “electronic compensation”, hereinafter referred to as “compensation”.
The net shallow impurity concentration in a semiconductor is defined as the absolute value of ND−NA, where ND and NA are the concentrations of shallow donors and acceptors, respectively. A semiconductor is fully compensated when the concentration of deep levels is higher than the net shallow impurity concentration. Doping with vanadium and introduction of deep point defects has been used for compensation of silicon carbide.
The solubility of boron in silicon carbide is very high (up to 0.1-0.5%), and unintentional boron acceptors can be present in sublimation-grown SiC crystals at levels as high as 1·1018 cm−3. In order to achieve reliable compensation, the concentration of deep levels should be higher than this level. In general, introduction of deep levels in high concentrations is technologically difficult and can cause stress and generation of defects. A better approach to full compensation would be through a reduced presence of unintentional shallow impurities, including boron.
Physical Vapor Transport (PVT) is the most common sublimation technique used for SiC crystal growth. A schematic diagram of the conventional PVT arrangement is shown in FIG. 1. Generally, growth is carried out in a graphite crucible 1 sealed with a graphite lid 2 and loaded with a sublimation source 3 and a seed 4. Generally, a polycrystalline SiC source 3 is disposed at the bottom of the crucible 1 and a SiC seed 4 at the top of crucible 1. The seed 4 is often mounted directly to the crucible lid 2 using adhesives or other suitable means. Crucible 1 is heated to a growth temperature, generally between 2000° C. and 2400° C., where source 3 vaporizes and fills crucible 1 with volatile molecular species of Si2C, SiC2 and Si. During growth, the temperature of source 3 is maintained higher than the temperature of the seed 4. This temperature difference forces the vapors from source 3 to migrate and precipitate on seed 4 forming a single crystal 5. In order to control the growth rate and ensure high crystal quality, PVT growth is carried out under a small pressure of inert gas, generally between several and 200 Torr.
It is known that the permeability of graphite depends on the nature of the gas diffusing through graphite. Graphite is generally permeable to inert gases, hydrogen and nitrogen, but has a much lower permeability to the elements that form stable carbides. Accordingly, graphite has a very low permeability to the vapors formed during sublimation of silicon carbide, such as Si, Si2C and SiC2. Therefore, conventional PVT can be viewed as a “closed” process, in which the Si-bearing vapors practically do not leave the growth crucible, except small unintentional losses that can occur through the joint between the crucible body 1 and lid 2.
SiC single crystals have also been grown using “open” processes, where a deliberate gas flow was established between the crucible interior and exterior. Examples include High Temperature Chemical Vapor Deposition (HTCVD), Halide Chemical Vapor Deposition (HCVD) and some PVT modifications. A generalized diagram of the open SiC growth process is shown in FIG. 2. Similarly to the closed sublimation growth process, the open process is carried out in a graphite crucible 1, wherein source 3 is disposed at the crucible bottom and seed 4 is disposed at the crucible top. Graphite crucible 1 used in the open process is provided with a gas inlet 7 and gas outlet(s) 9. A gas mixture 6, that may contain Si precursors, C precursors, dopants and other gaseous components, enters the crucible through an inlet 7. Once inside the crucible 1, the reactants undergo chemical transformations in a reaction zone 8. The gaseous reaction products blend with the vapors originating from solid source 3 and move toward seed 4, where they precipitate on seed 4 and form single crystal 5. Gaseous byproducts escape through gas outlet(s) 9. In the process of Halide Chemical Vapor Deposition (HCVD), the silicon and carbon precursors are delivered to the reaction zone 8 in the form of silicon tetrachloride (SiCl4) and propane (C3H8) mixed with a large excess of hydrogen. The main drawback of the open SiC sublimation growth process is related to severe losses of Si-bearing vapors through the outlet port(s) 9.
Graphite is widely utilized in SiC sublimation growth as a material for crucibles, seed-holders, heat shields and other parts. The starting materials used in graphite manufacturing (coke and pitch) contain boron. Therefore, boron is always present in graphite, where its atoms are chemically bound to carbon. High-temperature treatment under a halogen-containing atmosphere is widely used by graphite manufacturers for purification. During purification, the halogen molecules penetrate the graphite bulk, react with various impurities and form volatile halides with them. Driven by the concentration gradient, the halide molecules diffuse from the graphite bulk toward the surface, where they are removed by the flow of the carrier gas. Typically, removal of metallic impurities from graphite is more efficient than removal of boron.
Conventionally, graphite manufacturers characterize graphite purity by the “ash content”, i.e., the amount of ash that remains after a graphite specimen is burnt in oxygen. The best-purity commercially available graphite contains between 5 and 20 ppm of ash by weight. Boron forms volatile oxide upon burning in oxygen; therefore, graphite manufacturers seldom specify boron content in graphite. Impurity analyses using Glow Discharge Mass Spectroscopy (GDMS) show that the boron content even in the lowest-ash graphite is, typically, above 0.2 ppm and, in some cases, up to 1 ppm.
Furnaces used for graphite purification are, typically, very large and capable of accommodating metric tons of graphite. Cross-contamination between different items in a large graphite batch and contamination from the furnace itself limit the purification efficiency. As a result of the above limitations, graphite with a boron content below 0.1 ppm by weight is not readily available on a regular commercial basis.
Optimization of conventional PVT sublimation growth, including protective coatings applied to interior surfaces of a graphite crucible, has led to the reduction of boron in the grown SiC crystals to (2-3)·1016 cm−3. However, in order to produce semi-insulating SiC crystals of better quality and with superior electrical parameters, the concentration of unintentional boron must be reduced to levels below 1016 cm−3.